The invention relates to satellite communications systems for relaying signals between a first ground-based station and a second ground-based station.
In most prior art satellite communications systems, a network station transmits signals to a satellite in a first frequency band called the feeder uplink. After receiving feeder uplink signals, the satellite converts them to a second frequency band and transmits them to the mobile phones. The mobile phones reply on a third frequency band which is received at the satellite and translated to a fourth frequency band called the feeder downlink for transmission to the network station. Thus, in the prior art, four distinct frequency bands were required for a satellite communications system.
In the IRIDIUM system, the aforementioned second and third frequency bands are the same. An IRIDIUM satellite receives signals from a network station in the first frequency band and translates them to be relayed using a timeslot on a carrier frequency in a second frequency band. An IRIDIUM mobile phone receives the signal relayed by the satellite in the allocated timeslot on the allocated carrier frequency in the second frequency band. After a guard time to allow satellite transmissions that are reflected from the earth to propagate beyond the satellite orbit, the IRIDIUM phone replies in a second timeslot on the same second frequency band. The IRIDIUM satellite receives and translates the reply to the fourth frequency band and relays it to a network station. Thus, the IRIDIUM system uses three frequency bands for a given network station to communicate with a given mobile phone. The first and fourth frequency bands in the IRIDIUM system can include optical frequencies for conveying signals from satellite to satellite but ultimately radio frequency feederlinks are used for communication between the orbiting satellites and a network station. In the IRIDIUM system, the network stations do not transmit signals overlapping mobile phone transmissions in the same frequency band in the same cell or beam.
The prior art of landmobile radio systems includes the technique known as xe2x80x9ctwo-frequency simplexxe2x80x9d. In two-frequency simplex, a first mobile station desirous of communicating with a second mobile station transmits on a frequency f1 to a base station repeater. The base or repeater station usually comprises an elevated, typically directional, antenna that can receive the weak mobile station""s signal, which the second mobile at ground level cannot receive directly. The repeater station then translates the received signal from f1 to f2 and retransmits it at f2 to the second mobile station. When the second mobile station wishes to reply, it transmits on f1 to the repeater, but not at the same time as the first mobile station. If this happens, it is known as xe2x80x9cdoublingxe2x80x9d and the signal became garbled in the prior art.
In trunked landmobile radio systems, a station desirous of transmitting transmitted first an xe2x80x9caccess requestxe2x80x9d burst to the repeater using a third frequency f3 or calling channel, and the repeater replied on a channel f4 with an xe2x80x9caccess grantxe2x80x9d message indicating the channel f1 to be used for transmitting the rest of the communication, only if another station in the same group or net was not already transmitting. Accidental xe2x80x9cdoublingxe2x80x9d could thus be prevented in the prior art of trunked landmobile radio systems. These systems operated on a xe2x80x9cpush-totalkxe2x80x9d basis, which is a simplex and not a duplex communications method. By using voice-operated switching or xe2x80x9cVOXxe2x80x9d instead of hand-operated transmit switches, the appearance of engaging in a duplex or telephone type of conversation can be provided, but VOX is not a perfect technique and occasionally fails to adapt fast enough to change of the speech direction. It imposes a discipline on the speakers to wait until the other person has completely finished talking before replying, which is not present in natural or telephone conversations.
An early military communications system called the Defense Satellite Communications System, or DSCS for short, comprised only two frequency bands for respectively transmitting to the satellite and receiving signals relayed from the satellite. The satellite merely performed a frequency translation and amplification of the received signals prior to retransmission. This type of satellite is known as a xe2x80x9cbent pipexe2x80x9d transponder. These satellites had single, whole-earth coverage antenna beams. More powerful versions of DSCS type satellites known as SKYNET satellites were built and launched by Philco-Ford (now LORAL corporation) and GEC-Marconi for the British Defense Department.
In the prior art DCSC and SKYNET systems, a first station desirous of communicating with a second station transmitted a spread-spectrum signal to the satellite in a first frequency band. The second station also replied to the first station by transmitting a signal to the satellite in the first frequency band which was received overlapping the first station""s signal at the satellite. The bent-pipe satellite translated the sum of the received first and second stations"" signals to the second frequency band and relayed them to the first and second station. The first station despread the received signal using the second station""s spread-spectrum transmission code thereby suppressing interference from other signals, including its own, to an extent limited by the spread-spectrum processing gain. The second station likewise decoded the first station""s signal by despreading the received signal using the first station""s spread-spectrum transmission code, thereby suppressing other interfering signals, including its own, to an extent limited by the spread-spectrum processing gain. Indeed, spread-spectrum was used in this prior art for the purpose of discriminating the desired signal from interference including own signal. Spread spectrum was also used however to obtain resistance to enemy jamming. In this prior art, no attempt was made to remove own signal interference by storing own signal in a delay memory for subtraction from the signal received later from the satellite. There was moreover no motivation to do so as own signal interference was only a small fraction of the total interference, which could include enemy jamming.
By contrast with the DSCS and SKYNET prior art, the present invention is directed to a civil communications system which does not contemplate hostile jamming. There is therefore no motivation automatically to select spread spectrum techniques. However, the use of spread-spectrum signals is one implementation of the current invention and differs from the above prior art in the subtraction of own interference at a network station so as better to decode weaker, portable-station signals transponded by the satellite.
In U.S. Pat. Nos. 5,151,919 and 5,218,619 to Applicant, methods are disclosed in CDMA systems to subtract stronger interfering signals before demodulating weaker signals. However, in the applications disclosed all signals comprised largely unknown symbols that had to be decoded first prior to subtraction. This was because the interfering signals did not originate at the same station as the receiving station, as is the case for the invention described below. The above patents incorporated herein by reference and certain of the mathematical techniques disclosed therein can be incorporated into the current invention for subtracting known signals by nulling in a transform domain, when using signals of the type contemplated therein.
A satellite communications system includes at least one orbiting satellite having an antenna, or antennas, for transmitting signals in a first frequency band and receiving signals in a second frequency band. The antenna or antennas may furthermore be multi-beam antennas with each of the multiple beams covering a particular service region or cell. The present system furthermore comprises, in each cell, a first ground-based station for communicating with one or more second ground-based stations in the same cell. The first ground-based station can for example be a network station connected to the public switched telephone network (PSTN) or the Internet, and the second ground-based station may be a portable wireless telephone. Alternatively, the first station may also be a wireless telephone desirous of communicating with a second, similar, wireless telephone.
According to the present invention, the first station and the second station transmit to each other simultaneously using a first frequency band which signals are received overlapping at the satellite. The received, overlapping signals are translated to a second frequency band and then transmitted by the satellite to both the first and the second station. The first station thus receives from the satellite the signal from the second station overlapped with the signal it transmitted itself T milliseconds earlier, where T is the round-trip propagation delay. The first station remembers in an electronic memory the signal it transmitted T milliseconds earlier and subtracts the remembered signal from the received signal to reveal the second station""s signal, which it then decodes without interference from its own signal. The second station may do likewise to decode signals transmitted by the first station and relayed by the satellite.
According to another aspect of the invention, one of the stations may be designated as a master station (e.g., the network station) and provide the standard for frequency and timing. The second station is designated as a slave station (e.g., the mobile satellite phone) and determines a relative delay between the first station""s signal relayed by the satellite and its own signal relayed by the satellite. The second station then adjusts its transmit timing such that its own signal has a desired time-alignment with the first station""s signal as relayed by the satellite. Adjusting transmit timing can for example mean advancing or retarding transmission of a TDMA burst. Determining a relative timing can, for example, be done by correlating the received signal both with its own signal and with a known symbol pattern or syncword embedded in the first station""s transmission. The first or second station may subtract its own signal by using the correlation between the received signal and its previously transmitted and memorized signal to properly scale the memorized signal prior to subtraction. Correlation, scaling, and subtraction may moreover be applied with different time-alignments between the received signal and the memorized signal so as to compensate for intersymbol interference or multi-path propagation effects.
In a second implementation of the invention, a single frequency band only may be employed. The first and second stations transmit a TDMA burst to the satellite timed to arrive simultaneously at the satellite as previously described. The satellite receives the signals and stores them in a delay line or memory and then replays the signals out of the memory and retransmits them in a different TDMA timeslot during which it is not receiving signals from the ground stations. The ground stations receive the retransmitted signals and decode them after subtracting their own signal content as before.
According to yet another aspect of the invention, a first station (e.g., the network station) is equipped with a much larger antenna than the second station (e.g., the portable station) and is thus able to receive signals from the satellite largely uncorrupted by radio noise. It also transmits signals using its larger antenna that are received at the satellite much more strongly than the second station""s signal and thus the signal relayed by the satellite contains a much greater proportion of the first station""s signal than the second station""s signal. The second station may not therefore need to subtract its now much smaller, own-signal content in order to decode the first station""s signal, however the second station may do so nonetheless. The first station receives a signal from the satellite containing a much lower second station signal, and thus must subtract its own, dominant signal to reveal the weaker, underlying second station signal, which it can decode nonetheless thanks to its larger antenna having reduced the significance of thermal noise. Successful communications in both directions are guaranteed by maintaining a satisfactory uplink signal-to-noise ratio for the weaker, second station""s signal as received at the satellite, and by guaranteeing a satisfactory downlink signal-to-noise ratio for the stronger first station""s signal as received at the second station after being relayed by the satellite, with the quality of the uplink from the first station and the downlink to the first station being guaranteed by its much larger antenna. The second implementation of the invention using Time Division Duplex and a single frequency band may also be used with first and second stations having different antenna sizes.
According to a further aspect of the invention, the satellite effects cross-coupling of signals between beams under control of one or more control stations. A signal received in a first timeslot on a first frequency in a first beam comprises overlapping signals from a first and second station in the first beam. The composite overlapping signal is translated to a second frequency and transmitted in a second beam to a third station. The third station decodes the stronger of the first and second stations"" signals, subtracts it from the composite signal, and then decodes the weaker underlying signal. Alternatively, the third station jointly demodulates the overlapping first and second stations"" signals and couples a selected signal or both to the PSTN. The third station also transmits a signal to the satellite using said first frequency again in said second beam. Alternatively, another channel in the same band as the first frequency may be used. The third station""s signal is received at the satellite and may be overlapped by a signal transmitted on the same frequency and timeslot by a fourth station in the second beam. The satellite translates the third and fourth stations"" overlapped signals from the first frequency band to the second frequency band and transmits them in the first beam. The first and/or second station receives the relayed signals in the first beam and decodes the stronger of the third or fourth stations"" signals. The third or fourth station may proceed to subtract the decoded, stronger signal and then decode the weaker of the third or fourth stations"" signals, or else the third or fourth station may jointly demodulate both signals. The third or fourth station may then couple either or both of the decoded signals to the PSTN, or alternatively terminate the signal in its own telephone earpiece.
The invention to be described may utilize VOX or xe2x80x9cdiscontinuous transmissionxe2x80x9d (DTX) for the purposes of saving battery power in a mobile station or saving satellite power; however, DTX is not relied upon as a means to reverse the direction of traffic flow, which can be full duplex. Full duplex traffic flow is maintained when practicing the invention even during the aforementioned xe2x80x9cdoublingxe2x80x9d, and full duplex traffic flow can be even more useful in non-voice modes such as packet data modes for exchanging computer data.